Vitamin D is produced naturally by the body in response to skin exposure to UVB sunlight, the highest energy ultraviolet radiation the passes through the ozone barrier in any substantial amount. However, there are a number of reasons why endogenous (within the body itself) production of vitamin D often does not satisfy the body’s need. First, vitamin D production in response to sunlight declines with age. Ironically, older people also tend to suffer some of the harshest consequences for vitamin D deficiency. Also, sunscreen and avoidance of sun exposure has become more popular due to growing concerns over skin cancer and other forms of damage. A sunscreen of SPF 8 cuts down on the production of vitamin D by about 95%. Many sunscreens are SPF 30 and above, which stop almost 100% of vitamin D generation.
The other source of vitamin D is through the diet. In 1923, a biochemist named Harry Steenbock discovered that irradiating many foods with ultraviolet light increased their levels of vitamin D. By the 1930’s, governments began to fortify milk with vitamin D in an attempt to eradicate rickets in children, caused by vitamin D deficiency. However, milk is pretty much the only major food with added vitamin D. Even other dairy products like yogurt and cheese are often left unfortified. Adding to the problem is the fact that these days many people no longer drink milk. Besides milk, eggs, fatty fish, beef liver, and mushrooms naturally supply vitamin D. Vitamin D supplements are also widely available in both health food stores and groceries.
Whether produced internally or consumed, vitamin D undergoes a number of conversion steps before becoming fully active in the body. First it is transported to the liver where it is hydroxylated (gets an oxygen and a hydrogen atom added to it) and then stored until it is needed. Under conditions of low calcium, the vitamin D is transported through the blood stream to the kidneys where it is again hydroxylated to form calcitriol, the final active form of vitamin D. Calcitriol then binds to a receptor in the nuclei of certain cells, causing the cells to increase their production of transport proteins that help absorb calcium from food through the intestinal wall. As you can see, vitamin D serves as the body’s messenger, carrying the news of low calcium levels to the cells capable of fixing the problem. Without adequate vitamin D levels, this signaling pathway can’t function properly, the cells never get the message, and as a result calcium is poorly absorbed.
Generally, a good goal is to consume 1,000 IU of vitamin D per day good goal for people aged 19-50, while 1,200 IU per day is appropriate for people over 50. As with all vitamins and especially the fat soluble bunch (vitamins A, E, D, and K), be sure to count all of your vitamin D sources in order to maintain a healthy intake level without going overboard. Vitamin D toxicity is rare, but somewhere between 2,000-10,000 IU/day for adults and around 1,000 IU/day for children, over a period of about six months, can cause serious problems. So make sure to reach your goal, but also be aware of your total vitamin D intake. Vitamin D can be the difference between strong bones and debilitating injury. Set yourself up for a lifetime of health, mobility, and vigorous activity with proper levels of vitamin D and calcium!